1. Field of the Present Invention
The present invention relates to a method of gray scale display which can be advantageously employed in a dot matrix type display device for use as a display in, for example, personal computers and word processors.
2. Description of the Prior Art
Generally, in dot matrix type display units having no gray scale display function, i.e., binary level display function, such as liquid crystal, electroluminescence, and plasma display devices, gray scale display is simulated by two known methods, i.e., area gray scale method, for example, dizza method and frame thinning method. The area gray scale method has a disadvantage because it reduces display resolution. The frame thinning method involves a difficulty becomes a multi-gray scale is displayed, the trouble of so-called "flicker" occurs with respect to picture elements displayed in medium gray scale. Especially in the case of displaying a large area in same medium gray scale, the flicker is more remarkable, thus causing deterioration in display quality level.
A gray scale display system for reduction of such flicker is disclosed in Japanese Patent Laid-Open Publication No. SHO 61-205983. In this gray scale display system, as shown in FIG. 22 shows, all picture elements on the screen are classified into two kinds, i.e., picture element .alpha.and picture element .beta.. Picture elements .alpha. and .beta. are generally switched in antiphase respectively.
A picture element .alpha. and a picture element .beta. are arrayed alternately in vertical and horizontal directions to form a zigzag grating configuration. Therefore, through antiphase display performance of the picture elements .alpha. and .beta., the trouble of flicker may be prevented.
In the above noted prior art arrangement, if an 8-gray scale display, for example, is made for each picture element, the lighting mode of picture elements .alpha. in individual gray scales is shown in FIG. 23. In FIG. 23, (1) through (8) correspond respectively to gray scales of duty 8/8, 7/8, 6/8, 5/8 4/8, 3/8, 2/8 and 0/8. In FIG. 23, the high level denotes the lighting status of the picture element and the low level denotes the non-lighting status of the picture element. The term "duty" referred to herein means a rate of the number of frames in which a picture element is lighted up in one period to the total number of frames constituting the period, where the lighting mode of the picture element in each gray scale comprises 8 frames as the period, for example.
The lighting mode of picture element .beta. is shown in FIG. 24, in which (1) through (8) correspond respectively to the eight gray scales as above noted. FIG. 25 is a composite representation showing the lighting mode of picture elements .alpha. and .beta. in their individually corresponding gray scales. That is, it shows a lighting mode visually recognized when the picture elements .alpha. and .beta. are seen at same time. In FIG. 25, (1) through (8) also individually correspond to the gray scales shown. As can be seen from FIG. 25, gray scale display is not satisfactory in smoothing effect with respect to gray scales of duty 2/8 and 6/8, as also in gray scales of duty 3/8, 5/8, and 7/8. For example, if the frequency of vertical synchronizing signal is 70 Hz, a luminance variation of 17.5 Hz will occur with the gray scales of duty 3/8, 5/8, and 7/8, and a luminance variation of 35 Hz with the gray scales of duty 2/8 and 6/8. Such high luminance variation may not be reduced even if the area for display in these gray scales is increased. Especially where an area having more than a certain expanse of the screen, is displayed in the gray scale of duty 3/8, therefore, a flicker of 17.5 Hz is particularly remarkable and adversely affects the quality level of display. With such a method, therefore, it has been difficult to achieve multi-gray scale display without causing a deterioration in the quality level of display in all gray scales involved in the multi-gray scale display.